Pope has a good sugestion about double checking flatness and proper composition gaskets. Make sure (doubly sure) the meter block is super clean inside especially. Compressed air to gently blow out the passages in the booster passages on the body and anything thet appears to be a "hole". I am curious about the rapid vacuum fluctuation though. This is an indication of mecahnic problems. Possibly valves being to tight. How fast was the gauge flucuating?. If it was gradual up and down, it could still be a cause of improper fuel transition through the carb. If it's fairly rapid, Look for the mechanical things.